Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
  • B22F3/12—Both compacting and sintering
  • B22F3/14—Both compacting and sintering simultaneously
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/04—Making non-ferrous alloys by powder metallurgy
  • C22C1/05—Mixtures of metal powder with non-metallic powder
  • C22C1/051—Making hard metals based on borides, carbides, nitrides, oxides or silicides; Preparation of the powder mixture used as the starting material therefor

Definitions

• the present invention relates to a sintering process for liquid phase sintering of powder metallurgical parts, and the like, to close tolerances without warpage.
• U.S. Pat. No. 4,431,605 describes a method for densifying previously sintered parts of powdered materials, etc.
• the parts may be sintered in either vacuum or hydrogen and may be cooled in a similar atmosphere.
• the parts are then reheated and the sintering vessel is pressurized to densify the parts.
• AT 314212 discloses a method of sintering powder metallurgical parts according to which high pressure is applied after the eutectic temperature of the binder phase has been reached.
• the liquid phase wets the solid particles rapidly and forms isolated pores within the structure which then is characterized by closed porosity. It has been found that a certain shrinkage occurs already during this heating. However, the remaining porosity and flaws still make the mechanical properties after cooling inferior compared to fully dense materials. In normal low-pressure sintering, final densification needs a long time for complete pore elimination, which occurs through vacancy diffusion and annihilation.
• the present invention comprises a method of liquid phase sintering of powder metallurgical parts in a high pressure furnace comprising applying a pressure to the part of from about 0.1-100 MPa at a temperature below the formation of eutectic liquid phase, T liq , and maintaining this pressure during the remainder of the sintering cycle until the part has been subjected to substantial cooling.
• the present invention comprises a method in which porous powder metallurgical compacts, or the like, are placed inside a pressurizable vessel with a heating device.
• the compacts are heated in vacuum, inert gas or reducing protective atmosphere at approximately atmospheric or less pressure.
• a high pressure in the order of 0.1 to 100 MPa, preferably 0.3-30 MPa strongly accelerates the pore closure in a powder metallurgical part if the pressure is applied at a temperature which is lower, generally 2-50, preferably 5-30, most preferably 10-20, °C. lower, than that at which the liquid eutectic phase is formed, T liq .
• the T liq temperature varies depending upon the material. Typically, said temperature is in the range of 1200° C.-1600° C.
• a higher pressure has to be used if the material to be sintered has a low content of liquid phase, ⁇ 10 mol-% or fine grain size ⁇ 1 ⁇ m.
• the pressure is maintained during the rest of the sintering cycle until the part has been subjected to substantial cooling, generally until the the furnace has cooled to almost room temperature or until it is cooled to at least 800° C.
• a pressure cycle with increasing or decreasing pressure may be used.
• the process of the present invention may be performed in a conventional high pressure furnace.
• the invention applies in particular to powder metallurgical parts comprising at least one hard constituent comprising a carbide, nitride and/or carbonitride of at least one metal of groups IVB, VB and/or VIB of the Periodical System and a binder metal based on Co, Ni and/or Fe.
• the explanation for the rapid pore closure is that the solid grains are forced by the external pressure to move into the most energetically favorable positions and this movement is strongly assisted by the low-viscosity binder phase. At the same time, the dissolution of carbide phase is facilitated thus forming still more binder phase which makes the final densification extremely rapid. It has also been found that the binder phase exhibits a typical pseudoplastic behavior in the temperature range in question.
• the pressurizing is made at the right temperature, i.e., before the temperature has been reached at which the liquid binder phase is formed.
• this temperature is defined as the eutectic temperature at equilibrium, the pressurizing has to be made below this temperature.
• the eutectic temperature varies according to the composition and deviation from stoichiometric composition of a tungsten carbide-cobalt alloy. It is also known that this temperature is lower when the carbon content is high in the alloy than if it is low.
• the stoichiometric carbon content can be calculated from the formula:
• ⁇ C carbon excess or deficiency with respect to the stoichiometric content according to the above formula.
• the difference may be called ⁇ C and may be several tenths of a percent, positive or negative, and is often intentionally used in the powder composition as a means for correction of the final sintered product composition to obtain specific properties.
• the starting temperature for pressurizing the sintering vessel must be in a temperature range of T liq -50 to T liq -2, preferably T liq -30 to T liq -5, most preferably T liq -20 to T liq -10, °C.
• T liq can be determined experimentally, e.g., by differential thermal analysis (DTA).
• DTA differential thermal analysis
• T liq For straight tungsten carbide-cobalt alloys, T liq can be calculated from the formula:
• the alloy also contains other carbides such as of the fourth and fifth group of the transition elements in the Periodic Table of the elements, e.g., titanium carbide, niobium carbide and/or tantalum carbide, a corresponding correction to T liq has to be made.
• such correction terms are positive.
• the dwell time depends inter alia on the thinnest dimension of the compact, e.g., the thickness of a plate-shaped insert or the diameter of a long rod or the wall thickness of a tube or equivalent. If this dimension is called d (millimeters) the dwell time t (minutes) can be calculated from the formula:
• the method according to the present invention can also be used to deliberately change the carbon content of cemented carbide pieces using the improved transport capability of carbon reactive gases. For example, it may be necessary to correct the carbon balance obtained from initially used raw materials. Furthermore, the process can also be intentionally used to obtain requirements of mechanical or other physical properties of the sintered cemented carbide within narrow limits. If such corrections are necessary, carbon active gases such as hydrogen, methane, carbon monoxide, carbon dioxide, ammonia or water vapor may be used instead of inert gases or partly substituted for the inert gas.
• carbon active gases such as hydrogen, methane, carbon monoxide, carbon dioxide, ammonia or water vapor may be used instead of inert gases or partly substituted for the inert gas.
• nitrogen-containing materials such as titanium carbonitride-based alloys
• nitrogen or nitrogen-containing gas mixtures or gaseous compounds as a reactive pressurizing medium. This is especially important as many nitrides tend to disproportionate at high temperatures and thereby lose their valuable high temperature properties.
• the method according to the invention can also be used for the successful production of high speed steel according to powder metallurgy methods.
• the wear resistance of such materials can also be appreciably improved by mixing the high speed steel powder with wear resistant particles of, e.g., nitrides, such as titanium nitride or cubic boron nitride.
• Inserts of different styles, types VBMM, CNMM and TNMG were made from cemented carbide powders of two grades by uniaxial compaction to 55% relative density. The grades were:
• the sintered inserts were polished for metallographic examination and inspected for porosity according to ISO 4505.
• the high pressure sintered inserts according to the invention were absolutely pore free corresponding to A00 according to ISO 4505.
• the conventionally sintered inserts showed a porosity of A02, A04 or even worse in some instances for both grades.
• Test pieces for transverse rupture strength (TRS) determination were pressed to 55% relative density from powder of the two grades of Example 1.
• the test pieces were divided into two groups. One group was high pressure sintered according to the invention and the other group was conventionally sintered according to the conditions of Example 1. Testing of the transverse rupture strength was made according to ISO 3327. The following results were found:
• Test pieces for transverse rupture (TRS) determination were pressed to 55% relative density from powder of WC-6%Co with a carbon content of 5.61 weight-%.
• the test pieces were divided into two groups. One group was high pressure sintered according to the invention and the other group was conventionally sintered according to the conditions of Example 1. Density, porosity, K IC and hardness (HV10) were determined. The followinq results were found:

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Manufacturing & Machinery (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Powder Metallurgy (AREA)
• Press Drives And Press Lines (AREA)
• Ceramic Products (AREA)

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• 1990
  • 1990-11-05 SE SE9003521A patent/SE9003521D0/sv unknown
• 1991
  • 1991-11-01 US US07/786,608 patent/US5151247A/en not_active Expired - Lifetime
  • 1991-11-04 AT AT91850271T patent/ATE137695T1/de not_active IP Right Cessation
  • 1991-11-04 EP EP91850271A patent/EP0485353B1/en not_active Expired - Lifetime
  • 1991-11-04 DE DE69119361T patent/DE69119361T2/de not_active Expired - Fee Related

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